Wood-boring Beetle Parasitoids

Last week I was sitting with some friends around a campfire, making soup in a hollowed-out pumpkin.  One of them was whittling away at a piece of a red maple branch when, suddenly, she got to this:

I have my friends trained pretty well, and she knew it was time to hand over that chunk of wood and look for something else to whittle.  She had uncovered some tunnels of wood-boring beetles, but there should only be one larva per tunnel, and there are six visible in this one (each about 3 mm long).  Presumably they are wasp larvae that have parasitized the beetle larva that made the tunnel–or else parasitized something else that had appropriated the tunnel.  I covered them over with some sawdust and put the chunk of wood in the fridge for the winter, so with any luck I’ll be able to report back on what they turn out to be in the spring.

Meanwhile, I’m still slowly sorting through all the photos I took this past year, and last night I got to this one, taken in northern Vermont on June 28 (which means I’m now exactly five months behind):

This 5-mm beetle reminded me a little of a tumbling flower beetle (Mordellidae), so I guessed it was a member of some related family (Tenebrionoidea), but I couldn’t find a match.  I posted it on BugGuide and went to bed.  In the morning, I had my answer: it’s Pelecotoma flavipes, a wedge-shaped beetle (Ripiphoridae).  Wedge-shaped beetles are among the few beetles that parasitize other insects.  Relatively little is known about them, but as far as is known, members of the subfamily Ripiphorinae parasitize bees and wasps, their young larvae waiting for their hosts on flowers just as I observed a blister beetle larva doing this spring; the Ripidiinae parasitize cockroaches; and the Pelecotominae parasitize wood-boring beetles.  The European species Pelecotoma fennica has been studied in detail, and the habits of P. flavipes are presumed to be similar.  Females lay eggs at the openings of tunnels of death-watch beetles (Anobiidae) in dead or dying trees, and upon hatching the larvae enter the tunnels in search of their hosts.  The Pelecotoma larva bores into a death-watch beetle larva and stays inside until the following spring, when it emerges and attaches itself to the outside of its host.  It then grows rapidly, entirely consuming its host and then pupating in its host’s pupal chamber, emerging as an adult in late spring or early summer. (This information comes from Arnett et al. 2002, American Beetles, Vol. 2.)

The beetle in my photo is a female; males have fancy antennae.  Pelecotoma flavipes is known to parasitize Ptilinus ruficornis, another species in which males have fancy antennae.